OFDM基础外文资料翻译可编辑.docx

OFDM基础外文资料翻译可编辑.docx

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OFDM基础外文资料翻译可编辑

OFDM基础--外文资料翻译

附录A外文资料

OFDMBasics

INTRODUCTION

ThebasicprincipleofOFDMistosplitahigh-ratedatastreamintoanumberoflower

ratestreamsthataretransmittedsimultaneouslyoveranumberofsubcarriers.Becausethesymboldurationincreasesforthelowerrateparallelsubcarriers,therelativeamountofdispersionintimecausedbymultipathdelayspreadisdecreased.IntersymbolinterferenceiseliminatedalmostcompletelybyintroducingaguardtimeineveryOFDMsymbol.Intheguardtime,theOFDMsymboliscyclicallyextendedtoavoidintercarrierinterferenceInOFDMsystemdesign,anumberofparametersareupforconsideration,suchasthenumberofsubcarriers,guardtime,symbolduration,subcarrierspacing,modulationtypepersubcarrier,andthetypeofforwarderrorcorrectioncoding.Thechoiceofparametersisinfluencedbysystemrequirementssuchasavailablebandwidth,requiredbitrate,tolerabledelayspread,andDopplervalues.Somerequirementsareconflicting.Forinstance,togetagooddelayspreadtolerance,alargenumberofsubcarrierswithasmallsubcarrierspacingisdesirable,buttheoppositeistrueforagoodtoleranceagainstDopplerspreadandphasenoiseGENERATIONOFSUBCARRIERSUSINGTHEIFFT

AnOFDMsignalconsistsofasumofsubcarriersthataremodulatedbyusingphaseshiftkeyingPSKorquadratureamplitudemodulationQAM.IfdiarethecomplexQAMsymbols,Nisthenumberofsubcarriers,Tisthesymbolduration,andfisthecarrierfrequency,thenoneOFDMsymbolstartingattt,canbewrittenas2.1

Intheliterature,oftentheequivalentcomplexbasebandnotationisused,whichisgivenby2.2.Inthisrepresentation,therealandimaginarypartscorrespondtothein-phaseandquadraturepartsoftheOFDMsignal,whichhavetobemultipliedbyacosineandsineofthedesiredcarrierfrequencytoproducethefinalOFDMsignal.Figure2.1showstheoperationoftheOFDMmodulatorinablockdiagram.

2.2Figure2.1OFDMmodulator

Asanexample,Figure2.2showsfoursubcarriersfromoneOFDMsignal.Inthisexample,allsubcarriershavethesamephaseandamplitude,butinpracticetheamplitudesandphasesmaybemodulateddifferentlyforeachsubcarrier.NotethateachsubcarrierhasexactlyanintegernumberofcyclesintheintervalT,andthenumberofcyclesbetweenadjacentsubcarriersdiffersbyexactlyone.Thispropertyaccountsfortheorthogonalitybetweenthesubcarriers.Forinstance,ifthejthsubcarrierfrom2.2isdemodulatedbydownconvertingthesignalwithafrequencyofj/TandthenintegratingthesignaloverTseconds,theresultisaswrittenin2.3.Bylookingattheintermediateresult,itcanbeseenthatacomplexcarrierisintegratedoverTseconds.Forthedemodulatedsubcarrierj,thisintegrationgivesthedesiredoutputmultipliedbyaconstantfactorT,whichistheQAMvalueforthatparticularsubcarrier.Forallothersubcarriers,theintegrationiszero,becausethefrequencydifferenceproducesanintegernumberofcycleswithintheintegrationintervalT,suchthattheintegrationresultisalwayszero.

2.3

TheorthogonalityofthedifferentOFDMsubcarrierscanalsobedemonstratedinanotherway.Accordingto2.1,eachOFDMsymbolcontainssubcarriersthatarenonzerooveraT-secondinterval.Hence,thespectrumofasinglesymbolisaconvolutionofagroupofDiracpulseslocatedatthesubcarrierfrequencieswiththespectrumofasquarepulsethatisoneforaT-secondperiodandzerootherwise.TheamplitudespectrumofthesquarepulseisequaltosincnJT,whichhaszerosforallfrequenciesfthatareanintegermultipleof1IT.ThiseffectisshowninFigure2.2,whichshowstheoverlappingsincspectraofindividualsubcarriers.Attheimumofeachsubcarrierspectrum,allothersubcarrierspectraarezero.BecauseanOFDMreceiveressentiallycalculatesthespectrumvaluesatthosepointsthatcorrespondtotheimaofindividualsubcarriers,itcandemodulateeachsubcarrierfreefromanyinterferencefromtheothersubcarriers.Basically,Figure2.3showsthattheOFDMspectrumfulfillsNyquist'scriteriumforanintersymbolinterferencefreepulseshape.Noticethatthepulseshapeispresentinthefrequencydomain

andnotinthetimedomain,forwhichtheNyquistcriteriumusuallyisapplied.Therefore,insteadofintersymbolinterferenceISI,itisintercarrierinterferenceICIthatisavoidedbyhavingtheimumofonesubcarrierspectrumcorrespondtozerocrossingsofalltheothers.

Figure2.2ExampleoffoursubcarrierswithinoneOFDMsymbol

ThecomplexbasebandOFDMsignalasdefinedby2.2isinfactnothingmorethantheinverseFouriertransformofN,QAMinputsymbols.ThetimediscreteequivalentistheinversediscreteFouriertransformIDFT,whichisgivenby2.4,wherethetimetisreplacedbyasamplenumbern.Inpractice,thistrans